Experimental and numerical analysis in heat flow sensors calibration

The 2010/31/CE directive has highlighted the necessity to improve the energy efficiency in buildings, since they are responsible for 40% of energy consumption and 36% of CO2 emissions in the EU. In situ measurements of envelope components are needed for estimating the thermal transmittance of existing buildings in order to perform the energy certification. The instruments devoted to this aim, the heat flux meters (HFMs), are unfortunately not always provided with metrological traceability, and the appropriate reference standards are not available. The calibration systems currently adopted present different limits related to low performances at low and constant heat fluxes, non-uniformity of the heat flux in the measurement section and impossibility to control the heat flux at different temperatures. In this paper, the authors discuss the metrological behaviour of an existing HFM reference standard. A numerical model is employed to analyse the calibration system in different operating conditions (low and moderate heat fluxes), to improve it and to design a new prototype presenting high performances also in the presence of the HFM under calibration. In particular, the numerical tool is applied to investigate the heat flux uniformity and the metrological performances in a specific sub-region of the measuring section where the HFM under calibration is applied. A detailed experimental analysis is also conducted with the objective to validate the adopted numerical tool. Moreover, a metrological characterization of the system affords a combined standard uncertainty of better than 6% at low heat flows.